Stable, moderately unsaturated distillate fuel blend stocks prepared by low pressure hydroprocessing of Fischer-Tropsch products

ABSTRACT

The invention relates to a distillate fuel comprising a stable, low sulfur, highly paraffinic, moderately unsaturated distillate fuel blend stock. The highly paraffinic, moderately unsaturated distillate fuel blend stock exhibits excellent combustion properties in diesel and jet engines as a result of the high paraffin content. The blend stock is preferably prepared from a Fischer-Tropsch derived product that is hydroprocessed under conditions during which a moderate amount of unsaturates are formed or retained.

RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. application Ser. No. 09/999,667, “Distillate Fuel Blends from Fischer Tropsch Products with Improved Seal Swell Properties,” filed Oct. 19, 2001, the contents of which are hereby incorporated by reference. The present application is also related to U.S. patent application Ser. No. ______(Docket No. 005950-779) entitled “Highly Paraffinic, Moderately Aromatic Distillate Fuel Blend Stocks Prepared by Low Pressure Hydroprocessing of Fischer Tropsch Products,” filed herewith.

FIELD OF THE INVENTION

The invention is directed to a low sulfur distillate fuels comprising a Fischer-Tropsch distillate fuel blend stock, which have excellent stability and moderate content of unsaturates.

BACKGROUND OF THE INVENTION

Distillate fuel derived from the Fischer-Tropsch process is highly paraffinic and has excellent burning properties and very low sulfur. This makes Fischer-Tropsch products ideally suited for fuel use where environmental concerns are important. Fuels with good or preferably excellent stabilities are always desired, and stable fuels can be produced by hydrotreating or hydrocracking Fischer Tropsch products. However, conventional hydrotreating and hydrocracking processes require the use of expensive hydrogen to saturate olefins and convert oxygenates into paraffins.

Stable diesel fuels with low sulfur contents and high cetane indexes, are desired because of their low emissions and good engine performance. Likewise stable jet fuels with low sulfur contents and high smoke points are desired. Fuels of this type can be prepared from Fischer-Tropsch products. The preparation of distillate fuels from Fischer Tropsch processes is well known.

While they are highly paraffinic, Fischer-Tropsch products also contain olefins, alcohols, and traces of other compounds that can cause problems with stability. Typically, hydroprocessing is used to saturate essentially all the olefins and remove oxygenates. However, hydroprocessing requires the use of expensive hydrogen gas and expensive facilities designed to operate at high pressure.

ASTM specifications for Diesel Fuel (D985) describe stability measurements for the respective fuels. For diesel fuel, ASTM D6468, “Standard Test Method for High Temperature Stability of Distillate Fuels” is under consideration as a standard test method for a diesel fuel and this test can provide a good measure of the stability of the fuel. Neat hydrotreated and hydrocracked Fisher Tropsch products typically have excellent stabilities in this test. ASTM specifications for Jet Fuel A and A-1 (D1655) describe the use of the JFTOT test ASTM D3241 with a pass at 260° C. Higher stabilities are often desired. For example, Colonial Pipeline's Quality Assurance guidelines from February 2003 (section 3.19.1 on page 3B-33) require fungible aviation kerosene to have a JFTOT stability of 275° C. or greater. This higher stability requirement will provide some compensation for degradation in the stability of the product during shipping. The Coordinating Research Council's Handbook of Aviation Fuel Properties, Third Printing, May 1988 describes on page 102 (Table 10) that several military jet fuels have higher stability requirements than commercial Jet A: JP-9 and JP-10 require a stability of 300° C. or greater, JP-7 and TS require a stability of 335° or greater. Highly stable jet fuels are in general desirable.

In addition to conventional measurements of stability (thennal and storage), studies by Vardi et al (J. Vardi and B. J. Kraus, “Peroxide Formation in Low Sulfur Automotive Diesel Fuels,” February 1992, SAE Paper 920826) describe how fuels can develop significant levels of peroxide during storage, and how these peroxides can attack fuel system elastomers (O-rings, hoses, etc.). The formation of peroxides can be measured by Infrared spectroscopy, chemical methods, or by the attack on elastomer samples. As described by Vardi et al, fuels can become unstable with respect to peroxide formation when their sulfur content is reduced to low levels by hydroprocessing. Vardi et al also describe how compounds like tetralin can cause fuels to become unstable with respect to peroxide formation, while polycyclic aromatic compounds like naphthalenes can improve stability. Vardi et al. explains that aromatics act as natural antioxidants and notes that natural peroxide inhibitors such as sulfur compounds and polycyclic aromatics can be removed.

Following on the work by Vardi, two recent patents from Exxon describe how the peroxide-stability of highly-paraffinic Fischer Tropsch products in unacceptable, but can be improved by the addition of sulfur compounds from other blend components. However, since sulfur compounds increase sulfur emissions, this approach is not desirable.

By way of example, U.S. Pat. No. 6,162,956 discloses a Fischer-Tropsch derived distillate fraction blended with either a raw gas field condensate distillate fraction or a mildly hydrotreated condensate fraction to obtain a stable, inhibited distillate fuel. The fuel is described as a blend material useful as a distillate fuel or as a blending component for a distillate fuel comprising: (a) a Fischer-Tropsch derived distillate comprising a C₈−700° F. fraction, and (b) a gas field condensate distillate comprising a C₈−700° F. fraction, wherein the sulfur content of the blend material is ≧1 ppm by wt. This patent discloses that distillate fuels derived from Fischer-Tropsch processes are hydrotreated to eliminate unsaturated materials, e.g., olefins, and most, if not all, oxygenates. This patent further discloses that the products contain less than or equal to 0.5 wt % unsaturates (olefins and aromatics).

Similarly, U.S. Pat. No. 6,180,842 discloses a Fischer-Tropsch derived distillate fraction blended with either a raw virgin condensate fraction or a mildly hydrotreated virgin condensate to obtain a stable inhibited distillate fuel. The fuel is describes as a blend material useful as a distillate fuel or as a blending component for a distillate fuel comprising (a) a Fischer-Tropsch derived distillate comprising a C₈−700° F. stream and having a sulfur content of less than 1 ppm by wt, and (b) 1-40 wt % of a virgin distillate comprising a C₈−700° F. stream; wherein the sulfur content of the blend material is ≧2 ppm by wt. This patent notes that while there is no standard for the peroxide content of fuels, there is general acceptance that stable fuels have a peroxide number of less than about 5 ppm, preferably less than about 4 ppm, and desirably less than about 1 ppm. This value is tested after storage at 60° C. in an oven for 4 weeks. The patent shows that Fischer Tropsch products having a peroxide number of 24.06 after 4 weeks have unacceptable stability.

The Fischer Tropsch products in the '842 patent are described as being >80 wt %, preferably >90 wt %, more preferably >95 wt % paraffins, having an iso/normal ratio of 0.1 to 10, preferably 0.3 to 3.0, more preferably 0.7 to 2.0; sulfur and nitrogen of less than 1 ppm each, preferably less than 0.5, more preferably less than 0.1 ppm each; ≦0.5 wt % unsaturates (olefins and aromatics), preferably ≦0.1 wt %; and less than 0.5 wt % oxygen on a water free basis, preferably less than about 0.3 wt % oxygen, more preferably less than 0.1 wt % oxygen and most preferably nil oxygen. The '842 patent teaches that the Fischer Tropsch distillate is essentially free of acids.

U.S. Pat. No. 5,689,031 demonstrates that olefins in low-sulfur diesel fuel contribute to peroxide formation. See Fuels C and D in Example 7, and FIG. 2. The '031 patent teaches that the solution to the peroxide forming tendency is to limit the olefin content by hydrotreating the lightest olefin fraction. However, this solution requires the use of expensive hydrogen gas.

Accordingly, there is a need in the art for low sulfur distillate fuels and distillate fuel blend stocks with satisfactory stability, which can be obtained from Fischer-Tropsch process products while minimizing the use of expensive hydrogen. This invention provides such distillate fuels and distillate fuel blend stocks and the processes for their manufacture.

SUMMARY OF THE INVENTION

The present invention relates to a distillate fuel comprising a Fischer-Tropsch distillate fuel blend stock. The Fischer Tropsch distillate fuel blend stock comprises unsaturates in an amount between 2 and 20 weight %, paraffins in an amount 80 weight % or greater, sulfur in an amount less than 1 ppm, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks, and the Fischer Tropsch distillate fuel blend stock has a cetane index of greater than 60.

In another embodiment, the present invention relates to a Fischer-Tropsch diesel fuel blend stock. The Fischer Tropsch diesel fuel blend stock comprises unsaturates in an amount between 2 and 20 weight %, paraffins in an amount 90 weight % or greater, sulfur in an amount less than 1 ppm and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The unsaturates comprise less than 20 weight % polynuclear aromatics, preferably less than 10 weight % polynuclear aromatics, and even more preferably less than 5 weight % polynuclear aromatics. Preferably the unsaturates comprise both olefins and aromatics, and most preferably the olefins are present in amounts greater than or equal to 1 wt %. Characteristics of the diesel fuel blend stock include a cetane index greater than 60, a percent reflectance according to ASTM D6468 at 150° C. in excess of 65% when measured at 90 minutes.

In yet another embodiment, the present invention relates to a Fischer-Tropsch jet fuel blend stock. The Fischer Tropsch jet fuel blend stock comprises unsaturates in an amount between 2 and 10 weight %, paraffins in an amount 90 weight % or greater, sulfur in an amount less than 1 ppm, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The unsaturates comprise less than 20 weight % polynuclear aromatics, preferably less than 5 weight % polynuclear aromatics, and even more preferably less than 5 weight % polynuclear aromatics. Characteristics of the jet fuel blend stock include a smoke point of 30 mm or greater, and a passing rating in ASTM D3241 (JFTOT Procedure) at 260° C. for 2.5 hours.

In a further embodiment, the present invention relates to a process for preparing a highly paraffinic, moderately unsaturated distillate fuel blend stock. The process comprises converting syngas to a Fischer Tropsch derived feedstock by a Fischer Tropsch process and hydroprocessing the Fischer-Tropsch derived feedstock. A highly paraffinic, moderately unsaturated distillate fuel blend stock is recovered. The highly paraffinic, moderately unsaturated distillate fuel blend stock contains between 2 and 20 weight % unsaturates less than 1 ppm sulfur, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The hydroprocessing conditions include a temperature of 600-750° F., a pressure of less than 1000 psig, and a liquid hourly space velocity of greater than 0.25 hr⁻¹.

In yet a further embodiment the present invention relates to a distillate fuel comprising a Fischer Tropsch distillate fuel blend stock, wherein the Fischer Tropsch distillate fuel blend stock is made by a process comprising converting syngas to a Fischer Tropsch derived feedstock by a Fischer Tropsch process; hydroprocessing the Fischer-Tropsch derived feedstock at a temperature of 525-775° F., a pressure of less than 1000 psig, and a liquid hourly space velocity of greater than 0.25 hr⁻¹; and recovering a Fischer Tropsch distillate fuel blend stock. The Fischer Tropsch distillate fuel blend stock, which is recovered, comprises between 2 and 20 weight % unsaturates, less than 1 ppm sulfur, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks.

In a further embodiment, the present invention relates to a method of operating a diesel engine comprising using a Fischer Tropsch diesel fuel blend stock as a diesel fuel wherein the Fischer Tropsch diesel fuel blend stock comprises unsaturates in an amount between 2 and 20 weight %, paraffins in an amount 90 weight % or greater, sulfur in an amount less than 1 ppm and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The unsaturates comprise less than 20 weight % polynuclear aromatics, preferably less than 10 weight % polynuclear aromatics, and even more preferably less than 5 weight % polynuclear aromatics. Preferably the unsaturates comprise both olefins and aromatics, and most preferably the olefins are present in amounts greater than or equal to 1 wt %. Characteristics of the diesel fuel blend stock include a cetane index greater than 60, a percent reflectance according to ASTM D6468 at 150° C. in excess of 65% when measured at 90 minutes.

In yet another embodiment, the present invention relates to a method of operating a jet engine comprising using a Fischer Tropsch jet fuel blend stock as a jet fuel wherein the Fischer Tropsch jet fuel blend stock comprises unsaturates in an amount between 2 and 10 weight %, paraffins in an amount 90 weight % or greater, sulfur in an amount less than 1 ppm, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The unsaturates comprise less than 20 weight % polynuclear aromatics, preferably less than 5 weight % polynuclear aromatics, and even more preferably less than 5 weight % polynuclear aromatics. Characteristics of the jet fuel blend stock include a smoke point of 30 mm or greater, and a passing rating in ASTM D3241 (JFTOT Procedure) at 260° C. for 2.5 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is an illustration of a process to make a distillate fuel blend stock according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it has been found that low sulfur, highly paraffinic, moderately unsaturated distillate fuels blend stocks can be produced, which have excellent stability. The distillate fuel blend stocks can be prepared by a process including a Fischer-Tropsch synthesis and hydroprocessing under conditions in which a moderate amount of unsaturates are formed or are retained in the product. Accordingly, the present invention relates to low sulfur, highly paraffinic, moderately unsaturated distillate fuel blend stocks that have excellent stability and distillate fuels comprising these blend stocks. The low sulfur, highly paraffinic, moderately unsaturated distillate fuel blend stocks can be mixed with other blend stocks to provide a distillate fuel or can be used neat directly in an engine as a fuel in the absence of other blend stocks with only the optional addition of minor amounts of additives.

For purposes of the present invention, the following definitions will be used herein:

The term “unsaturates” mean a hydrocarbon containing one or more double or triple bonds, preferably, for example, aromatic and/or olefinic functionality.

The term “olefins” means an unsaturated straight or branched chain hydrocarbon having at least one double bond (i.e., an alkene).

The term “aromatics” means an unsaturated, cyclic and planar hydrocarbon with an uninterrupted cloud of electrons containing an odd number of pairs of π electrons.

A “distillate fuel blend stock” is a material that is mixed with other distillate fuel blend stocks to provide a distillate fuel, in particular a diesel or jet fuel, as herein defined. The blend stock itself does not necessarily meet specifications for the respective fuel, but preferably the resulting combination of blend stocks does. Jet fuel blend stocks are combined with other jet fuel blend stocks, and optionally additives, to provide jet fuel. Similarly, diesel fuel blend stocks are combined with other diesel fuel blend stocks, and optionally additives, to provide diesel fuel.

An “aromatic blend stock” is a blend stock comprising aromatics in an amount greater than or equal to 50 weight %, preferably greater than or equal to 75 weight %, and most preferably greater than or equal to 90 weight %. If a pure aromatic product is used as an aromatic blend stock, analysis of the aromatic content is not necessary. If the aromatic blend stock comprises aromatics and other hydrocarbons, a modified version of ASTM D6550 (Standard Test Method for the Determination of the Olefin Content of Gasolines by Supercritical Fluid Chromatography (SFC)) can be used to determine the aromatics. An aromatic blend stock may be blended with a Fischer Tropsch distillate fuel blend stock to increase the aromatic content of the Fischer Tropsch distillate fuel blend stock. Examples of aromatic blend stocks include commercially available pure aromatics (for example, benzene, alkylbenzenes, and the like); aromatics obtained from conventional petroleum products; aromatics obtained from reforming of reformable Fischer-Tropsch products; and the like.

The Cetane Index was determined by ASTM D4737-96a(2001) Standard Test Method for Calculated Cetane Index by Four Variable Equation.

Conventional petroleum products comprise products derived from petroleum crude.

A “petroleum blend stock” is a blend stock that comprises conventional petroleum products. Petroleum blend stocks may be comprised of the vapor overhead streams from distilling petroleum crude or refined products and the residual fuels that are the non-vaporizable remaining portion.

Derived from a Fischer-Tropsch process means that the feedstock, blend stock, or product in question originates from or is produced at some stage by a Fischer-Tropsch process.

A “Fischer Tropsch distillate fuel blend stock” is a blend stock that originates or is produced at some stage by a Fischer Tropsch process. A Fischer Tropsch distillate fuel blend stock can be mixed with other distillate fuel blend stocks to provide a distillate fuel, in particular a diesel or jet fuel. The blend stock itself does not necessarily meet specifications for the respective fuel, but the resulting combination of blend stocks does. Fischer Tropsch distillate fuel blend stocks include Fischer Tropsch diesel fuel blend stocks and Fischer Tropsch jet fuel blend stocks. As stated above, the Fischer Tropsch distillate fuel blend stocks can be mixed with other blend stocks to provide a distillate fuel, or the Fischer-Tropsch distillate fuel blend stocks can be used neat directly in an engine as a fuel in the absence of other blend stocks with only the optional addition of minor amounts of additives.

A distillate fuel is a material containing hydrocarbons with boiling points between approximately 60° F. to 1100° F. Within the broad category of distillate fuels are specific fuels including naphtha, jet fuel, diesel fuel, kerosene, aviation gas, fuel oil, and blends thereof.

A diesel fuel is a material suitable for use in diesel engines. Preferably, a diesel fuel conforms to at least one of the following specifications:

-   -   ASTM D975—“Standard Specification for Diesel Fuel Oils”     -   European Grade CEN 90     -   Japanese Fuel Standards JIS K 2204     -   The United States National Conference on Weights and Measures         (NCWM) 1997 guidelines for premium diesel fuel     -   The United States Engine Manufacturers Association recommended         guidelines for premium diesel fuel (FQP-1A)         A diesel fuel may be comprised of a combination of blend stocks         or a single blend stock in the absence of other blend stocks         with only the optional addition of minor amounts of additives.

A jet fuel is a material suitable for use in turbine engines in aircraft or other uses. Preferably, a jet fuel conforms to at least one of the following specifications:

-   -   ASTM D1655,     -   DEF STAN 91-91/3 (DERD 2494), TURBINE FUEL, AVIATION,     -   KEROSENE TYPE, JET A-1, NATO CODE: F-35,     -   International Air Transportation Association (IATA) Guidance         Materials for Aviation, 4th edition, March 2000         A jet fuel may be comprised of a combination of blend stocks or         a single blend stock in the absence of other blend stocks with         only the optional addition of minor amounts of additives.

A Fischer Tropsch diesel fuel blend stock is a blend stock suitable for use in a diesel engine. The Fischer Tropsch diesel fuel blend stock may be mixed with other blend stocks to provide a diesel fuel or may be used in the absence of other blend stocks with only the optional addition of minor amounts of additives.

A Fischer Tropsch jet fuel blend stock is a blend stock suitable for use in turbine engines in aircraft or other uses. The Fischer Tropsch jet fuel blend stock may be mixed with other blend stocks to provide a jet fuel or may be used in the absence of other blend stocks with only the optional addition of minor amounts of additives.

A highly paraffinic, moderately unsaturated distillate fuel blend stock is a distillate fuel blend stock that contains more than 70 weight % paraffins, 80 weight % or greater paraffins, and most preferably 90 weight % or greater paraffins and 2 to 20 weight % unsaturates, preferably 2 to 15 weight % unsaturates, and most preferably 5 to 10 weight % unsaturates. Preferably the unsaturates comprise both olefins and aromatics, and most preferably the olefins are present in amounts greater than or equal to 1 wt %. A low sulfur highly paraffinic, moderately unsaturated distillate fuel blend stock contains less than 1 ppm sulfur. Preferably, the highly paraffinic, moderately unsaturated distillate fuel blend stock is a Fischer Tropsch distillate fuel blend stock.

A “Fischer-Tropsch derived feedstock” or “Fischer Tropsch feedstock” is a feedstock that originates from or is produced at some stage by a Fischer Tropsch process. In the processes of the present invention, a Fischer-Tropsch derived feedstock may be blended with a petroleum blend stock during processing to provide a blended stream.

Syngas is a mixture that includes both hydrogen and carbon monoxide. In addition to these species, water, carbon dioxide, unconverted light hydrocarbon feedstock and various impurities may also be present.

“Hydrocarbonaceous” or “hydrocarbon” means a compound or substance that contains hydrogen and carbon atoms, but which can include heteroatoms such as oxygen, sulfur or nitrogen.

“Peroxide precursors” mean those components in a hydrocarbon product or feed that will form peroxides and/or cause formation of peroxides in the hydrocarbon product. The peroxide precursors can be identified and the amount of peroxide precursors can be measured by storage of the hydrocarbon product in an oven at 60° C. for 4 weeks. The peroxide precursors are identified by the formation of peroxides and the amount of peroxide precursors can be measured based on the amount of peroxides formed. According to the present processes, the peroxide content of samples is measured by use of a procedure for measuring the buildup of peroxides as described in ASTM D3703. By way of example, a 4 ounce sample is placed in a brown bottle and aerated for 3 minutes. After the testing period, an aliquot of the sample is then tested according to ASTM D3703 with the exception that isooctane is used in place in freon. Tests confirmed that the substitution of solvents for environmental reasons had no significant effect on the measurement results. The formation of peroxides can also be measured by Infrared spectroscopy, chemical methods, or by attack on elastomer samples.

A modified version of ASTM D6550 (Standard Test Method for the Determination of the Olefin Content of Gasolines by Supercritical Fluid Chromatography (SFC) was used to determine the group types in the feedstocks and products. The modified method uses a 3-point calibration standard to quantify the total amount of saturates, aromatics, oxygenates (polars) and olefins. Calibration standard solutions were prepared using the following compounds: undecane, toluene, n-octanol and dodecene. External standard method was used for quantification and the detection limit for aromatics and oxygenates is 0. 1% wt and for olefins is 1.0% wt. ASTM D6550 describes the instrument conditions.

A small aliquot of the fuel sample was injected onto a set of two chromatographic columns connected in series and transported using supercritical carbon dioxide as the mobile phase. The first column was packed with high surface area silica particles. The second column contained high surface area silica particles loaded with silver ions.

Two switching valves were used to direct the different classes of components through the chromatographic system to the detector. In a forward-flow mode, saturates (normal and branched alkanes and cyclic alkanes) were passed through both columns to the detector, while olefins were trapped on the silver-loaded column and the aromatics and oxygenates are retained on the silica column. Aromatic compounds and oxygenates were subsequently eluted from the silica column to the detector in a back flush mode. Finally, the olefins were back flushed from the silver-loaded column to the detector.

A flame ionization detector (FID) was used for quantification. Calibration was based on the area of the chromatographic signal of saturates, aromatics, oxygenates and olefins, relative to standard reference materials, which contain a known mass % of total saturates, aromatics, oxygenates and olefins as corrected for density. The total mass collected was 100%+/−3%, and was normalized to 100% for convenience.

The Polynuclear Aromatic (PNA) content of the products was determined by ASTM D5186-99 Standard Test Method for Determination of Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels and Aviation Turbine Fuels by Supercritical Fluid Chromatography.

The paraffin content of the product was determined by the Supercritical Fluid Chromatography (SFC) analysis, using the following algorithm. The SFC analysis provides determinations of aromatics, olefins, oxygenates and saturates. Saturates in this analysis are a combination of paraffins and naphthenes (i.e. cycloparaffins). Thus, Paraffins=Saturates (SFC)−Naphthenes However, naphthenes were not found to be present in significant amounts (less than 10% of the saturates) in the products of the present invention. Thus, saturates from the SFC analysis typically can be taken as a good and proper measurement of the paraffin content of the products of the invention.

To verify that naphthenes are not present in significant amounts, the naphthene content was determined independently using GC-MS. GC-MS reports olefins and cycloparaffins as a combined sum because they have the same ratio of hydrogen to carbon in their structures and the technique cannot distinguish them. If GC-MS reports the combined sum of the olefins and cycloparaffins as being insignificant, then the naphthenes can be concluded to be present only in insignificant amounts. However, if GC-MS reports the sum as being significant, the portion of naphthenes can be determined by subtracting the olefin content (as determined by SFC) from the combined GC-MS sum to provide the naphthenes. Naphthenes=(Sum of Naphthenes and Olefins by GC-MS)−(Olefins by SFC) The naphthenes can then be subtracted from the saturates content (as determined by SFC) to provide a good and proper measurement of the paraffin content according to the first equation. If the naphthene content determined in this way is less than zero, it is reported as zero, and zero is used in the calculation of paraffins. Thus, in this case the paraffins are equal to the SFC saturates.

In the GC-MS test, deuterium labeled standards were used to quantify alkanes, olefins, alcohols, and acids. Selected deuterium labeled compounds were added to the sample of interest as internal standards. The mixture of sample and standards were treated with trimethylsilyl (TMS) reagent to form the TMS derivatives, followed by GCMS analysis. The mass spectrometer is a Hewlett-Packard bench top mass spectrometer interfaced to a HP GC with a 60 meter non-polar column. The normal alkanes and the branched alkanes were all quantified using deuterium labeled normal alkanes. Olefins, alcohols, and acids were all quantified using the corresponding deuterium labeled compounds.

The paraffin content of the highly paraffinic, moderately unsaturated blend stocks of the present invention is at least 70 weight %, preferably 80 weight % or greater, and most preferably 90 weight % or greater. Due to their high content of paraffins, the highly paraffinic, moderately unsaturated distillate fuel blend stocks of the present invention have excellent combustion properties. Characteristic combustion properties of the blend stocks of the present invention include smoke points in excess of 25 mm, preferably in excess of 30 mm, and cetane indexes in excess of 60, preferably in excess of 65. The paraffins consist of a mixture of normal and iso-paraffins with the ratio of iso/normal paraffins in the fuel being between 0.3 and 10. Higher proportions of iso-paraffins are preferred when the blend stock is intended for use in cold climates (Jet A1 or diesel for arctic use).

The unsaturates content of the blend stocks of the present invention is between 2 and 20 weight %, preferably between 2 and 15 weight %, and most preferably between 5 and 10 weight %. The unsaturates of the blend stocks comprise minimal amounts of polynuclear aromatics. Preferably, the unsaturates comprise less than 25 weight % polynuclear aromatics, preferably less than 20 weight % polynuclear aromatics, more preferably less than 10 weight % polynuclear aromatics, and even more preferably less than 5 weight % polynuclear aromatics. Preferably the unsaturates comprise both olefins and aromatics, and most preferably the olefins are present in amounts greater than or equal to 1 weight %.

Fuels comprising the highly paraffinic, moderately unsaturated blend stocks of the present invention preferably conform to at least one specification for either diesel or jet fuel. The fuels may be comprised of a combination of blend stocks or the highly paraffinic, moderately unsaturated blend stock in the absence of other blend stocks with only the optional addition of minor amounts of additives. The highly paraffinic, moderately unsaturated blend stocks and fuels comprising this blend stock exhibit at least acceptable, and most often excellent, stability. For example, the percent reflectance of the diesel fuels comprising the highly paraffinic, moderately unsaturated blend stocks, as measured by ASTM D6468 at 150° C., will be in excess of 65% when measured at 90 minutes, preferably in excess of 65% when measured at 180 minutes, and more preferably in excess of 99% when measured at 180 minutes. Jet fuels comprising the highly paraffinic, moderately unsaturated blend stocks have a passing rating in ASTM D3241 (JFTOT Procedure) at 260° C. for 2.5 hours, preferably a passing rating in ASTM D3241 (JFTOT Procedure) at 270° C. for 2.5 hours, and more preferably a passing rating in ASTM D3241 (JFTOT Procedure) at 300° C. for 2.5 hours. A passing rating corresponds to a tube rating of less than 3 (Code 3) and a pressure drop across a filter of less than 25 mm Hg.

The blend stocks of the present invention, and fuels comprising the blend stocks, display acceptable stability according to the conventional tests of stability and acceptable peroxide resistance. The blend stocks form less than 5 ppm peroxides after storage at 60° C. in an oven for 4 weeks, preferably less than 4 ppm peroxides after storage at 60° C. in an oven for 4 weeks, and more preferably less than1 ppm peroxides after storage at 60° C. in an oven for 4 weeks. Accordingly, the blend stocks comprise peroxide precursors in such an amount that the fuel forms less than 5 ppm peroxides after storage at 60° C. in an oven for 4 weeks, preferably less than 4 ppm peroxides after storage at 60° C. in an oven for 4 weeks, and more preferably less than1 ppm peroxides after storage at 60° C. in an oven for 4 weeks. The amount of peroxide precursors is measured by storage in an oven at 60° C. in an oven for 4 weeks. The amount of peroxide precursors can be determined based on the amount of peroxides formed. The formation of peroxides can be measured by Infrared spectroscopy, chemical methods, or by attack on elastomer samples.

The blend stocks of the present invention, and fuels comprising the blend stocks, typically have low sulfur (<1 ppm) and preferably low nitrogen content (<1 ppm). Therefore, environmental emissions of oxides of heteroatoms are minimized. Accordingly, the blend stocks and fuels comprising the blend stocks are desirable as environmentally friendly.

Fischer-Tropsch

The blend stocks of the present invention can be prepared from Fischer-Tropsch products, hydroprocessed under conditions in which a moderate amount of unsaturates are formed or retained. Preferably, the blend stocks of the present invention are at least partially derived from a Fischer-Tropsch process.

In Fischer-Tropsch chemistry, syngas is converted to liquid hydrocarbons by contact with a Fischer-Tropsch catalyst under reactive conditions. Typically, methane and optionally heavier hydrocarbons (ethane and heavier) can be sent through a conventional syngas generator to provide synthesis gas. Generally, synthesis gas contains hydrogen and carbon monoxide, and may include minor amounts of carbon dioxide and/or water. The presence of sulfur, nitrogen, halogen, selenium, phosphorus and arsenic contaminants in the syngas is undesirable. For this reason and depending on the quality of the syngas, it is preferred to remove sulfur and other contaminants from the feed before performing the Fischer-Tropsch chemistry. Means for removing these contaminants are well known to those of skill in the art. For example, ZnO guardbeds are preferred for removing sulfur impurities. Means for removing other contaminants are well known to those of skill in the art. It also may be desirable to purify the syngas prior to the Fischer-Tropsch reactor to remove carbon dioxide produced during the syngas reaction and any additional sulfur compounds not already removed. This can be accomplished, for example, by contacting the syngas with a mildly alkaline solution (e.g., aqueous potassium carbonate) in a packed column.

In the Fischer-Tropsch process, liquid and gaseous hydrocarbons are formed by contacting a synthesis gas comprising a mixture of H₂ and CO with a Fischer-Tropsch catalyst under suitable temperature and pressure reactive conditions. In general, a Fischer-Tropsch reaction can be conducted at temperatures from about 300 to 700° F. (149 to 371° C.), preferably from about 400 to 550° F. (204 to 228° C.); pressures of from about 10 to 600 psia, (0.7 to 41 bars), preferably 30 to 300 psia, (2 to 21 bars) and catalyst space velocities of from about 100 to about 10,000 cc/g/hr, preferably 300 to 3,000 cc/g/hr.

Fischer Tropsch processes can be categorized as either a high temperature Fischer Tropsch process or a low temperature Fischer Tropsch process. The process conditions and the predominate products from the two processes are different.

A high temperature Fischer Tropsch process is generally carried out at temperatures above 250° C., preferably at or above 350° C. High temperature Fischer Tropsch processes provide primarily lower molecular weight olefinic products generally within the C₃ to C₈ range, preferably propylene to pentenes. High temperature Fischer Tropsch products can also contain significant amounts of aromatics. The high temperature Fischer Tropsch products may be subjected to processes to saturate the aromatics, including reforming processes. The olefinic products from the high temperature Fischer Tropsch process are typically further processed by oligomerization and hydrogenation steps to produce a highly branched iso-paraffinic product. The products from the high temperature Fischer Tropsch process can be processed so that they meet specifications for gasoline. The products from high temperature Fischer Tropsch processes typically have cetane indexes of about 55 since the products are highly branched. An example of a high temperature Fischer Tropsch process is the Synthol process used by SASOL, as described in “High Yield High Quality Diesel from Fischer Tropsch Process, Dry, M. E., Chem. S. A., February 1984.

Jet fuels have also been produced by high temperature Fischer Tropsch processes, olefin oligomerization, and hydrogenation. A high temperature Fischer Tropsch process for making jet fuels is described in “Qualification of SASOL Semi-Synthetic Jet A-1 as Commercial Jet Fuel,” SwRI-8531, November 1997. The jet fuels made by a high temperature Fischer Tropsch process, as described in the reference, contain no aromatics or unsaturates. The thermal stability, or JFTOT, breakpoint for blends of high temperature Fischer Tropsch derived jet with conventional petroleum-derived is presented in the literature as in excess of 300° C. Therefore the thermal stability, or JFTOT, breakpoint for such semi-synthetic blends is significantly above the specification requirement of 260° C. See “Qualification of SASOL Semi-synthetic Jet A-1 as Commercial Jet Fuel”, Moses, Stavinoha, and Roets, South West Research Institute Publication SwRI-8531, November 1997.

Researchers working with high temperature Fischer Tropsch products and blends of high temperature Fischer Tropsch products and petroleum-derived components have not noted problems with stability.

A low temperature Fischer Tropsch process operates at temperatures below 250° C. and produces a heavier product. The heavier product of a low temperature Fischer Tropsch process commonly contains predominantly wax. The products from the low temperature Fischer Tropsch process are typically hydrotreated so that they will have acceptable peroxide stability, as shown by U. S. Pat. No. 6,180,842. Accordingly, the products from low temperature Fischer Tropsch processes are typically refined by hydroprocessing operations such as hydrotreating and hydrocracking to provide stable fuels meeting the desired specification. The products from low temperature Fischer Tropsch processes are predominantly linear, and even after hydrocracking, these products contain fewer branches than products made from a high temperature Fischer Tropsch process. Fewer branches in the products of low temperature Fischer Tropsch processes provide higher cetane indexes for these products in comparison to the products from the high temperature processes, which have increased branching. The low temperature Fischer Tropsch products typically have cetane indexes of greater than 60, and preferably greater than 70.

The Fischer Tropsch process, for making the distillate fuel blend stocks according to the present invention, is a low temperature Fischer Process. Examples of conditions for performing low temperature Fischer-Tropsch type reactions are well known to those of skill in the art.

The products may range from C₁ to C₂₀₀₊ with a majority in the C₅-C₁₀₀₊ range. The reaction can be conducted in a variety of reactor types for example, fixed bed reactors containing one or more catalyst beds, slurry reactors, fluidized bed reactors, or a combination of different type reactors. Such reaction processes and reactors are well known and documented in the literature. Slurry Fischer-Tropsch processes, which is a preferred process in the practice of the invention, utilize superior heat (and mass) transfer characteristics for the strongly exothermic synthesis reaction and are able to produce relatively high molecular weight, paraffinic hydrocarbons when using a cobalt catalyst. In a slurry process, a syngas comprising a mixture of H₂ and CO is bubbled up as a third phase through a slurry in a reactor which comprises a particulate Fischer-Tropsch type hydrocarbon synthesis catalyst dispersed and suspended in a slurry liquid comprising hydrocarbon products of the synthesis reaction which are liquid at the reaction conditions. The mole ratio of the hydrogen to the carbon monoxide may broadly range from about 0.5 to 4, but is more typically within the range of from about 0.7 to 2.75 and preferably from about 0.7 to 2.5.

Suitable Fischer-Tropsch catalysts comprise on or more Group VIII catalytic metals such as Fe, Ni, Co, Ru and Re. Additionally, a suitable catalyst may contain a promoter. Thus, a preferred Fischer-Tropsch catalyst comprises effective amounts of cobalt and one or more of Re, Ru, Pt, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material, preferably one which comprises one or more refractory metal oxides. In general, the amount of cobalt present in the catalyst is between about 1 and about 50 weight percent of the total catalyst composition. The catalysts can also contain basic oxide promoters such as ThO₂, La₂O₃, MgO, and TiO₂, promoters such as ZrO₂, noble metals (Pt, Pd, Ru, Rh, Os, Ir), coinage metals (Cu, Ag, Au), and other transition metals such as Fe, Mn, Ni, and Re. Support materials including alumina, silica, magnesia and titania or mixtures thereof may be used. Preferred supports for cobalt containing catalysts comprise titania. Useful catalysts and their preparation are known and illustrative, but nonlimiting examples may be found, for example, in U.S. Pat. Nos. 4,568,663.

Certain catalysts are known to provide chain growth probabilities that are relatively low to moderate, and the reaction products include a relatively high proportion of low molecular (C₂₋₈) weight olefins and a relatively low proportion of high molecular weight (C₃₀₊) waxes. Certain other catalysts are known to provide relatively high chain growth probabilities, and the reaction products include a relatively low proportion of low molecular (C₂₋₈) weight olefins and a relatively high proportion of high molecular weight (C₃₀₊) waxes. Such catalysts are well known to those of skill in the art and can be readily obtained and/or prepared.

The products from low temperature Fischer-Tropsch reactions generally include a light reaction product and a waxy reaction product. The waxy reaction product (i.e. the wax fraction) includes hydrocarbons boiling above about 600° F. (e.g., vacuum gas oil through heavy paraffins), largely in the C₂₀₊range, with decreasing amounts down to C₁₀. Both the light reaction product and the waxy product are substantially paraffinic. The waxy product generally comprises greater than 70% normal paraffins, and often greater than 80% normal paraffins.

Hydroprocessing

Hydroprocessing, in general, is well known to those of skill in the art and includes such processes as hydrotreating, hydrocracking, hydrogenation, catalytic dewaxing, or combinations of these processes. Preferably, the hydroprocessing operation of the present invention achieves several purposes in one or several reactors, most preferably a single reactor. Among the purposes of the hydroprocessing are reducing, or preferably completely removing, heteroatoms such as nitrogen and sulfur. While conventional hydroprocessing typically removes unsaturates or greatly reduces their contents, the hydroprocessing of this invention retains at least a portion of the unsaturates or creates aromatics; yet yields a distillate product that has acceptable stability . Moreover, the hydroprocessing may increase the ratio of iso/normal paraffins in the distillate product. Additionally, the hydroprocessing may increase the production of distillate product by converting heavy species. Finally, the hydroprocessing may also be conducted under conditions that create or retain a moderate amount of unsaturates.

Hydroprocessing under conditions to create or retain unsaturates may reduce or eliminate net hydrogen consumption in the hydroprocessing process. While the addition of hydrogen is needed to begin the hydroprocessing process, if the aromatics formation is high enough, the amount of hydrogen produced in the process can exceed the amount of hydrogen added to the process. Accordingly, there may be a net hydrogen production from the hydroprocessing of the present invention, i.e. the net hydrogen consumption is less than zero.

Produced hydrogen can be used for a variety of purposes in a Gas-To-Liquid (GTL) facility. Among these purposes are hydrotreatment of Fischer-Tropsch streams to reduce or eliminate olefins and/or heteroatoms. Further, produced hydrogen may be reacted with CO₂ produced in the GTL process or recovered from a CO₂-containing gas source to reduce CO₂. The product from the CO₂-H₂ reaction can be CO or a Fischer-Tropsch product and the reaction can be conducted in the syngas generator. Produced hydrogen may also be used in fuel production as a fuel component that does not form CO₂ emissions. The produced fuel can be used to generate process heat, produce electrical energy, and/or distill/purify water.

Typical temperatures for the hydroprocessing of Fischer-Tropsch products to retain unsaturates or generate aromatics are 525-775° F., preferably 575-725° F. Typical pressures for this operation are less than 1000 psig, preferably less than 600 psig, and most preferably between 200 and 500 psig. Typical liquid hourly space velocities (LHSV) for this operation are greater than 0.25 hr⁻¹, preferably between 0.5 and 1.5 hr⁻¹. Typical hydroprocessing catalysts for this operation include catalysts for conventional hydroprocessing operations (described below) or catalysts for hydroisomerization dewaxing, preferably combinations of catalysts for hydroprocessing operations and hydroisomerization dewaxing are used as this combination is less expensive and also permits a concurrent reduction in the product pour point.

A conventional hydroprocessing catalyst may be used for generating aromatics and retaining unsaturates. Hydroprocessing catalysts which are particularly suited for generating aromatics are bifunctional catalysts, which contain both a hydrogenation function and an acidic function. The aromatics-forming hydroprocessing catalyst is contrasted, for example, from a conventional hydrotreating catalyst by the presence of the acidic function, since hydrotreating catalysts typically include a non-acidic support such as alumina.

The acidic function is preferably based on a mixture of at least two metal oxides of different valences. The preferred mixture of metal oxides includes SiO₂ and Al₂O₃; or Al₂O₃, SiO₂, and P₂O₅. The mixture of metal oxides may be prepared in such a way as to provide a high dispersion of at least a portion of the metal oxides between themselves, for example dispersion of the SiO₂ and Al₂O₃ on an atomic scale rather than separate phases of SiO2 and Al₂O₃. The presence of separate phases of SiO₂ and Al₂O₃ can be determined by an XRD examination. If all of the oxides are present as separate phases, the performance of the catalyst will be diminished. Examples of the acidic function composed of mixed metal oxides are zeolites, crystalline SAPO's, and co-precipitated SiO₂—Al₂O₃.

While halogens can be used in hydroprocessing catalyst as an acidic function, especially fluoride as in the form of a fluorided alumina, halogens are not preferred because they will slowly be stripped from the catalyst and can lead to corrosion of the reactor vessel.

The hydrogenation function in an aromatics-forming hydroprocessing catalyst comprises a metal. Suitable hydrogenation metals include Group VI metals such as Mo and/or W and Group VIII metals such as Ni or Co. These are present on the catalyst in sulfided form. Preferably, the hydrogenation metal is a noble metal, more preferably selected from the group consisting of Pt, Pd and mixtures thereof. These may be sulfided, but using them in the non-sulfided form is preferred.

Catalysts useful in hydroprocessing operations are well known in the art. Suitable catalysts include noble metals from Group VIIIA (according to the 1975 rules of the International Union of Pure and Applied Chemistry), such as platinum or palladium on an alumina or siliceous matrix, and Group VIIIA and Group VIB, such as nickel-molybdenum, cobalt-molybdenum, nickel-tungsten, or nickel-tin on an alumina or siliceous matrix. The non-noble metal (such as nickel-molybdenum) hydrogenation metals are usually present in the final catalyst composition as oxides, or more preferably or possibly, as sulfides when such compounds are readily formed from the particular metal involved. Preferred non-noble metal catalyst compositions contain in excess of about 5 weight percent, preferably about 5 to about 40 weight percent molybdenum and/or tungsten, and at least about 0.5, and generally about1 to about 15 weight percent of nickel and/or cobalt determined as the corresponding oxides. The noble metal (such as platinum) catalyst may contain in excess of 0.01 percent metal, preferably between 0.1 and 1.0 percent metal. Combinations of noble metals may also be used, such as mixtures of platinum and palladium.

The matrix components include some that have acidic catalytic activity. Ones that have activity include amorphous silica-alumina or may be a zeolitic or non-zeolitic crystalline molecular sieve. Examples of suitable matrix molecular sieves include zeolite Y, zeolite X and the so called ultra stable zeolite Y and high structural silica:alumina ratio zeolite Y. Suitable matrix materials may also include synthetic or natural substances as well as inorganic materials such as clay, silica and/or metal oxides such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia, and silica-magnesia zirconia. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays, which can be composited with the catalyst, include those of the montmorillonite and kaolin families. These clays can be used in the raw state as originally mined or initially subjected to calumniation, acid treatment or chemical modification. More than one catalyst type may be used in the reactor.

As stated above, hydroprocessing, in general, is well know to those of skill in the art and includes such processes as hydrotreating, hydrocracking, hydrogenation, catalytic dewaxing, or combinations of these processes. The hydroprocessing of the present invention upgrades the Fischer Tropsch derived feedstock by performing an operation selected from the group consisting of reducing the content of sulfur, nitrogen, and oxygen in the feedstock; reducing the content of olefins in the feedstock; increasing the ratio of iso/normal paraffins in the product to between 0.3 and 10; increasing the production of distillate fuel product by converting heavy species in the feedstock; and combinations thereof.

Typical hydrotreating conditions vary over a wide range. Typical pressures for this operation are less than 1000 psig, preferably less than 600 psig, and most preferably between 200 and 500 psig. Typical liquid hourly space velocities (LHSV) for this operation are greater than 0.25 hr⁻¹, preferably between 0.5 and 2.0 hr⁻¹. Hydrogen recirculation rates are typically greater than 50 standard cubic feet per barrel of oil (SCF/Bbl), and are preferably between 1000 and 5000 SCF/Bbl. Temperatures range from about 300° F. to about 750° F., preferably ranging from 450° F. to 600° F.

Hydrocracking may be conducted according to conventional methods known to those of skill in the art. Typically, hydrocracking is a process of breaking larger carbon molecules into smaller ones. It may be effected by contacting the particular fraction or combination of fractions, with hydrogen in the presence of a suitable hydrocracking catalyst at temperatures in the range of from about 600 to 900° F. (316 to 482° C.), preferably 650 to 850° F. (343 to 454° C.), and pressures in the range of from about 200 to 4000 psia (13 to 272 atm), preferably 500 to 3000 psia (34 to 204 atm) using space velocities based on the hydrocarbon feedstock of about 0.1 to 10 hr⁻¹, preferably 0.25 to 5 hr⁻¹. Generally, hydrocracking is utilized to reduce the size of the hydrocarbon molecules, hydrogenate olefin bonds, hydrogenate aromatics and remove traces of heteroatoms. Suitable catalysts for hydrocracking operations are known in the art and include sulfided catalysts. Sulfided catalyst may comprise amorphous silica-alumina, alumina, tungsten and nickel.

The conditions of hydrogenation are well known in the industry and include temperatures above ambient and pressures greater than atmospheric. Preferable conditions for hydrogenation include a temperature between 300 and 800° F., most preferably between 400 and 600° F., a pressure between 50 and 2000 psig, most preferably between 100 and 500 psig, a liquid hourly space velocity (LHSV) between 0.2 and 10 hr⁻¹, most preferably between 1.0 and 3.0 hr⁻¹, and a gas rate between 500 and 10,000 SCFB, most preferably between 1000 and 5000 SCFB.

The catalysts used for hydrogenation are those typically used in hydrotreating, but non-sulfided catalysts containing Pt and/or Pd are preferred, and it is preferred to disperse the Pt and/or Pd on a support, such as alumina, silica, silica-alumina, or carbon. The preferred support is silica-alumina.

Catalytic dewaxing consists of two main classes—conventional hydrodewaxing and hydroisomerization dewaxing; hydroisomerization dewaxing can be further subdivided into partial and complete hydroisomerization dewaxing. All classes involve passing a mixture of a waxy hydrocarbon stream and hydrogen over a catalyst that contains an acidic component to convert the normal and slightly branched iso-paraffins in the feed to other non-waxy species and thereby generate a lube base stock product with an acceptable pour point (below about +10° F. or −12° C.). Typical conditions for all classes involve temperatures from about 400 to 800° F., pressures from about 200 to 3000 psig, and space velocities from about 0.2 to 5 hr⁻¹. The method selected for dewaxing a feed typically depends on the product quality, and the wax content of the feed, with Conventional Hydrodewaxing generally preferred for low wax content feeds. The method for dewaxing can be effected by the choice of the catalyst. The general subject is reviewed by Avilino Sequeira, in Lubricant Base Stock and Wax Processing, Marcel Dekker, Inc pages 194-223.

The determination of the class of dewaxing catalyst among conventional hydrodewaxing, partial hydroisomerization dewaxing and complete hydroisomerization dewaxing can be made by using the n-hexadecane isomerization test as describe by Santilli et al. in U.S. Pat. No. 5,282,958. When measured at 96% n-hexadecane conversion under conditions described by Santilli et al., conventional hydrodewaxing catalysts will exhibit a selectivity to isomerized hexadecanes of less than 10%, hydroisomerization dewaxing catalysts will exhibit a selectivity to isomerized hexadecanes of greater than or equal to 10%, partial hydroisomerization dewaxing catalysts will exhibit a selectivity to isomerized hexadecanes of greater than 10% to less than 40%, and complete hydroisomerization dewaxing catalysts will exhibit a selectivity to isomerized hexadecanes of greater than or equal to 40%, preferably greater than 60%, and most preferably greater than 80%.

Conventional hydrodewaxing is defined for purposes of this document as a catalytic dewaxing process that uses a conventional hydrodewaxing catalyst. In conventional hydrodewaxing, the pour point is lowered by selectively cracking the wax molecules, mostly to smaller paraffins boiling between propane and about octane. Since this technique converts the wax to less valuable by-products, it is useful primarily for dewaxing oils that do not contain a large amount of wax. Waxy oils of this type are frequently found in petroleum distillate from moderately waxy crudes (Arabian, North Slope, etc). Catalysts that are useful for conventional hydrodewaxing are typically 12-ring zeolites and 10-ring zeolites. Zeolites of this class include ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, and Mordenite. Conventional hydrodewaxing catalysts favor cracking in comparison to other method of conversion of paraffins. This is demonstrated by use of the n-hexadecane isomerization test by Santilli et al., in which conventional hydrodewaxing catalysts exhibit a selectivity to isomerized hexadecane products of less than 10%. In addition to the zeolites, metals may be added to the catalyst, primarily to reduce fouling. Representative process conditions, yields, and product properties for conventional hydrodewaxing are described, for example, in U.S. Pat. Nos. 4,176,050 to Chen et al., 4,181,598 to Gillespie et al., 4,222,855 to Pelrine et al., 4,229,282 to Peters et al., and 4,211,635 to Chen. These patents are incorporated herein by reference for all purposes. Process conditions are further described and exemplified by Sequeira in the section titled “The Mobil Lube Dewaxing Process,” pages 198-204 and references therein, J. D. Hargrove, G. J. Elkes, and A. H. Richardson, Oil and Gas J., p. 103, Jan. 15, 1979.

Hydroisomerization dewaxing is defined for purposes of this document as a catalytic dewaxing process that uses a hydroisomerization dewaxing catalyst. Hydroisomerization dewaxing converts at least a portion of the wax to non-waxy iso-paraffins by isomerization, while at the same time minimizing the conversion by cracking. When conventional hydrodewaxing and hydroisomerization dewaxing are compared on the same feed, the conversion of wax to non-waxy iso-paraffins during hydroisomerization dewaxing gives benefits of reducing the yield of less valuable by-products, increasing the yield of lube oil, and generating an oil with higher VI and greater oxidation and thermal stability. Hydroisomerization dewaxing uses a dual-functional catalyst consisting of an acidic component and a metal component. Both components are required to conduct the isomerization reaction. Typical metal components are platinum or palladium, with platinum most commonly used. The choice and the amount of metal in the catalyst is sufficient to achieve greater than 10% isomerized hexadecane products in the test described by Santilli et al. When the selectivity for hexadecane isomers following Santilli's test exceed 40%, the catalyst is a complete hydroisomerization dewaxing catalyst. Since hydroisomerization dewaxing converts wax to iso-paraffins which boil in the lube base stock range, it is useful for dewaxing oils that contain a large amount of wax. Waxy oils of this type are obtained from slack waxes from solvent dewaxing processes, and distillates from highly waxy crudes (Minas, Altamont, etc.) and products from the Fischer-Tropsch process.

Partial hydroisomerization dewaxing is defined for purposes of this document as a catalytic dewaxing process that uses a partial hydroisomerization dewaxing catalyst. In partial hydroisoinerization dewaxing a portion of the wax is isomerized to iso-paraffins using catalysts that can isomerize paraffins selectively, but only if the conversion of wax is kept to relatively low values (typically below 70%). At higher conversions, wax conversion by cracking becomes significant, and yield losses of lube base stock becomes uneconomical. The acidic catalyst components useful for partial hydroisomerization dewaxing include amorphous silica aluminas, fluorided alumina, and 12-ring zeolites (such as Beta, Y zeolite, L zeolite). Because the wax conversion is incomplete, partial hydroisomerization dewaxing must be supplemented with an additional dewaxing technique, typically solvent dewaxing. The wax recovered from a solvent dewaxing operation following a partial hydroisomerization dewaxing can be recycled to the partial hydroisomerization dewaxing step.

Representative process conditions, yields, and product properties for partial hydroisomerization dewaxing are described, for example, in U.S. Pat. Nos. 5,049,536 to Belussi et al., and 4,943,672 to Hamner et al. These patents are incorporated herein by reference for all purposes. Process conditions are further described and exemplified in, EP 0 582 347 to Perego et al., EP 0 668 342 to Eilers et al., PCT WO 96/26993 by Apelian et al., and PCT WO 96/13563 by Apelian et al.

Complete hydroisomerization dewaxing is defined for purposes of this document as a catalytic dewaxing process that uses a complete hydroisomerization dewaxing catalyst. In complete hydroisomerization dewaxing, complete hydroisomerization dewaxing catalysts are used which can achieve high conversion levels of wax while maintaining acceptable selectivities to isomerization. The acidic catalyst components useful for partial hydroisomerization dewaxing include 10-ring, 1-dimensional, molecular sieves (such as ZSM-23, SSZ-32, Theta-1, ZSM-22, SAPO-11, and SAPO-41). Since wax conversion can be complete, or at least very high, this process typically does not need to be combined with additional dewaxing processes to produce a lube base stock with an acceptable pour point. Representative process conditions, yields, and product properties for complete hydroisomerization dewaxing are described, for example, in U.S. Pat. Nos. 5,135,638 to Miller; 5,246,566 to Miller; 5,282,958 to Santilli et al.; 5,082,986 to Miller; and 5,723,716 to Brandes et al.; the contents of each of which is incorporated herein by reference in their entirety. Catalytic Dewaxing Hydroisomerization Dewaxing Partial Complete Conventional Hydroisomerization Hydroisomerization Hydrodewaxing Dewaxing Dewaxing Temperature, ° F. 400-800 Pressure, psig 200-3000 LHSV, hr−1 0.2-5.0 n-C₁₆ selectivity to <10 10-40 >40 isomerized products at >60 preferably, 96% >80 most conversion preferably Typical Acidic ZSM-5, ZSM-11, Silica Alumina, ZSM-23, SSZ-32, Components ZSM-22, ZSM-35, Fluorided alumina, Theta-1, ZSM-22, Mordenite Beta, Y, and L SAPO-11, and zeolites SAPO-41 Typical metal Optional, often Pt or Pd, preferably Pt or Pd, preferably Components absent Pt Pt Blending with Petroleum Blend Stocks

The distillate fuel according to the present invention may be comprised of a combination of blend stocks, or the distillate fuel may be comprised of Fischer Tropsch distillate fuel blend stock in the absence of other blend stocks with only the optional addition of minor amounts of additives. Accordingly, the distillate fuels may comprise a Fischer-Tropsch distillate fuel blend stock mixed with petroleum blend stock. In a mixture of blend stocks, preferably the distillate fuel comprises1 to 95 weight % Fischer Tropsch blend stock and 5 to 99 weight % petroleum blend stock. More preferably, the distillate fuel comprises 5 to 75 weight % Fischer Tropsch blend stock and 25 to 95 weight % petroleum blend stock. Even more preferably, the distillate fuel comprises 10 to 50 weight % Fischer Tropsch blend stock and 90 to 50 weight % petroleum blend stock.

In addition, in the process to make a highly paraffinic, moderately unsaturated blend stock, the Fischer Tropsch feedstock may be blended with a petroleum blend stock at any stage in the process so long as a highly paraffinic, moderately unsaturated distillate fuel blend stock according to the present invention is provided. By way of example, a petroleum blend stock may be blended with a Fischer Tropsch derived feedstock prior to hydroprocessing, after hydroprocessing but prior to removing polynuclear aromatics, or after removal of polynuclear aromatics but prior to use as a distillate fuel. Preferably, the petroleum blend stock is blended with the Fischer Tropsch feedstock prior to hydroprocessing and the resulting blended stream is hydroprocessed. If the Fischer Tropsch feedstock is mixed with a petroleum blend stock, preferably, the resulting blend comprises1 to 95 weight % Fischer Tropsch feedstock and 99 to 5 weight % petroleum blend stock. More preferably, the blend comprises 5 to 75 weight % Fischer Tropsch feedstock and 95 to 25 weight % petroleum blend stock. Even more preferably, the blend comprises 10 to 50 weight % Fischer Tropsch feedstock and 50 to 90 weight % petroleum blend stock.

Removal of Polynuclear Aromatics

To meet the desired low content of polynuclear aromatics in the highly paraffinic, moderately unsaturated blend stock, the product stream from the hydroprocessing operation can be further treated to remove polynuclear aromatics. Options for selectively removing polynuclear aromatics from the product stream while leaving desired mono-aromatics, include selective hydrotreating and adsorption.

The most preferred operation for removing polynuclear aromatics from the product stream is selective hydrotreating. The reaction conditions for selective hydrotreating do not vary greatly from the reaction conditions for hydrotreating described above. Reaction conditions for selective hydrotreating include low temperatures (less than 750° F., preferably less than 700° F., most preferably less than 600° F.), high pressures (greater than 250 psig, preferably greater than 350 psig, most preferably greater than 500 psig), and short contact times (LHSV of less than 5 hr⁻¹, preferably less than 3 hr⁻¹, and most preferably less than 2 hr⁻¹). Preferred catalysts for this selective hydrotreating contain Pt, Pd, and combinations thereof. The selective hydrotreating will reduce the polynuclear aromatic content by at least 50 weight %, preferably at least 75 weight %, and most preferably at least 90 weight %, and the unsaturate content by less than 50 weight %, preferably less than 35 weight %, and most preferably less than 20 weight %.

The removal of polynuclear aromatics from the product stream can also be achieved by adsorption on an oxide support, preferably one that has moderate acidity (an acidic clay such as montmorillonite or attapulgite). The temperatures for adsorption should be less than 200° F., preferably less than 150° F. Polynuclear aromatics can also be extracted with a solvent, such as n-methyl pyrollidinone, phenol, or furfural.

Additives

The distillate fuel and distillate fuel blend stock may include additives that are commonly used for diesel or jet fuels. A description of diesel fuel additives that may be used in the present invention is as described in the Chevron Corporation, Technical Review Diesel Fuels, pp. 55-64 (2000) and a description of jet fuel additives that may be used in the present invention is as described in Chevron Corporation, Technical Review Aviation Fuels, pp 27-30 (2000). In particular, these additives may include, but are not limited to, antioxidants, lubricity additives, pour point depressants, and the like. The additives are added to the fuels and blend stocks in a minor amount, preferably less than 1 weight %.

ILLUSTRATIVE EMBODIMENT

The FIGURE represents a process for preparing a highly paraffinic, moderately unsaturated distillate fuel blend stock according to the present invention. With reference to FIG. 1, a low temperature Fischer-Tropsch derived feedstock (10) is hydroprocessed in a hydroprocessing unit (100) to which hydrogen (20) is added. Hydroprocessing conditions include a temperature of 600-750° F., a pressure of less than 1000 psig, and a liquid hourly space velocity of greater than 0.25 hr⁻¹. The product (30) of the hydroprocessing is a highly paraffinic, moderately unsaturated distillate fuel blend stock containing between 2 and 20 weight % unsaturates and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks. The hydroprocessing may produce hydrogen (60), which can be used in other processes such as hydrotreatment, CO₂ reduction, and fuel production. Optionally, the highly paraffinic, moderately unsaturated distillate fuel blend stock (30) may be treated to remove polynuclear aromatics (70) in a processing unit (200).

The following examples are given to illustrate the invention and should not be construed to limit the scope of the invention.

EXAMPLES Example 1

A light Low Temperature Fischer-Tropsch wax (Table I) was hydrocracked over a sulfided nickel-tungsten/silica-alumina catalyst (a conventional hydroprocessing catalyst), LHSV 1 hr⁻¹, 1000 psig, 685° F., and 6.3 MSCF/bbl. At these conditions, conversion below 650° F. was 80.4 weight %. The liquid product was cut at about 350° F. and about 675° F. to give a diesel blend stock fraction. Yields and properties of the diesel blend stock are given in Table II. TABLE I FEEDSTOCK INSPECTIONS OF LIGHT FT WAX Gravity, API 42.5 Nitrogen, ppm 3.2 Sim. Dist., LV %, ° F. ST/5 728/771 10/30 789/811 50 839 70/90 858/885 95/EP 898/943

TABLE II HYDROCRACKING OF LIGHT FT WAX OVER Ni—W—SIO₂—AL₂O₃ AT LHSV 1 hr⁻¹, 685° F., 1000 PSIG, AND 6.3 MSCF/BBL Conversion <650° F., Weight % 80.4 Yield, Weight % C₁-C₂  0.03 C₃-C₄  5.06 C₅-180° F. 17.77 180-350° F. 20.85 350-650° F. 37.51 650° F.+ 19.71 C₅₊ 95.49 350-675° F. Properties Weight % of Feed 52.9  Gravity, API 50.7  Viscosity, 40° C., cSt  2.631 Cloud Point, ° C. −26    SFC Analysis, Wt % Aromatics 0.3 Olefins 0.8 Oxygenates <0.1  Saturates 98.9  PNA Aromatics, weight % Not Detected Cetane Index 75.9  Refractive Index @ 20° C.   1.4342 Density, g/ml @ 20° C.   0.7745 Molecular Wt. 253    Carbon Types by ndM, weight % P/N/A 100/0/0 D2887 Dist., Wt %, ° F. ST/5 288/342 10/30 368/448 50 523    70/90 594/673 95/EP 697/743

The lower limit of detection of PNA by Supercritical Fluid Chromatography (SFC) is 0.5 weight %. Thus non-detected values are less than this amount. Operation at these conditions produced only 0.3 percent aromatics and retained 0.8 wt % olefins due to the high pressure conventional operation. The paraffin content of this sample is equivalent to the saturate content (98.9). The ndM analysis, which is suitable for non-olefin containing samples, indicates the absence of naphthenic carbon structures.

Example 2

The same feed as in Example1 was hydrocracked over a sulfided 3/1 layered bed of the same catalyst as in Example1 over a Pt/SAPO-11 catalyst, the latter bound with 15 weight % alumina. The Pt/SAPO-11 catalyst is a complete hydroisomerization dewaxing catalyst. Conditions were the same as in Example 3, that is overall LHSV 1.0 hr⁻¹, 1000 psig, 685° F., and 6.3 MSCF/bbl H₂. Conversion below 650° F. was 74.6 weight %. The product was cut at about 350° F. and about 650° F. to give a diesel blend stock cut. Yields and diesel blend stock properties are given in Table III. As determined by ASTM D6468, the diesel blend stock was very stable. Aromatics in the diesel blend stock were 0.1 weight % and olefins were 0.3 wt % due to the conventional high pressure operation and use of Pt as a catalytic metal. The paraffin content is 99.6 since naphthenes are absent as 20 determined by GC-MS, and supported by ndM analysis. The Cetane Index was very high (73.8) and the cloud point very low (−57° C.). TABLE III HYDROCRACKING OF LIGHT FT WAX OVER 3/1 Ni—W—SIO₂—AL₂O₃/PT-SAPO-11 AT LHSV 1 hr⁻¹, 685° F., 1000 PSIG, AND 6.3 MSCF/BBL Conversion <650° F., Weight % 74.6 Yield, Weight % C₁-C₂ 0.08 C₃-C₄ 5.16 C₅-180° F. 13.02 180-350° F. 15.70 350-650° F. 40.97 650° F.+ 25.59 C₅+ 95.36 350-650° F. Properties Weight % of Feed 43.1 Gravity, API 51.3 Viscosity, 40° C., cSt 2.206 Cloud Point, ° C. −57 Olefins + Naphthenes, Weight % (GC-MS) Not Detected PNA Aromatics, weight % Not Detected SFC Analysis, Wt % Aromatics 0.1 Olefins 0.3 Oxygenates <0.1 Saturates 99.6 % Reflectance, ASTM D6468 @ 150° C. 1.5 hr 99.7 3.0 hr 99.8 Cetane Index 73.8 Refractive Index @ 20° C. 1.4318 Density, g/ml @ 20° C. 0.7699 Molecular Wt. 239 D2887 Dist., Wt %, ° F. ST/5 314/352 10/30 370/433 50 496 70/90 549/606 95/EP 629/676

Example 3

Example 2 was repeated, but at a total pressure in the reactor of 500 psig, and a reactor temperature of 680° F. Conversion below 650° F. was 63.5 weight %. The product was cut at about 350° F. and about 590° F. to give a diesel blend stock cut. Yields and diesel blend stock properties are given in Table IV. As determined by ASTM D6468, the diesel blend stock was very stable. Aromatics in the diesel blend stock were 2.3 weight %. The Cetane Index was still quite high (69.1) and the cloud point very low (−50° C.). TABLE IV HYDROCRACKING OF LIGHT FT WAX OVER 3/1 Ni—W—SIO2—AL2O3/PT-SAPO-11 AT LHSV 1 hr⁻¹, 680° F., 500 PSIG, AND 6.3 MSCF/BBL Conversion <650° F., Weight % 63.5 Yield, Weight % C₁-C₂  0.23 C₃-C₄  10.53 C₅-180° F.  13.98 180-350° F.  15.63 350-650° F.  23.72 650° F.+  36.75 C₅+ 90.0 350-559° F. Properties Weight % of Feed 19.1 Gravity, API 51.1 Viscosity, 40° C., cSt  1.94 Cloud Point, ° C. −50   PNA Aromatics, weight % Not Detected % Reflectance, ASTM D6468 @150° C. 1.5 hr 99.7 3.0 hr 99.7 Cetane Index 69.1 Refractive Index @ 20° C.   1.4323 Density, g/ml @ 20° C.   0.7704 Molecular Wt. 224   D2887 Dist., Wt %, ° F. ST/5 316/350 10/30 366/415 50 468   70/90 519/572 95/EP 591/643

Example 4

A 700-1000° F. hydrotreated Low Temperature Fischer Tropsch wax (Table V) was hydrocracked over the same layered bed catalyst system of Example 2. Conditions included an overall LHSV 1.0 hr⁻¹, reactor pressure of 300 psig, 680° F. for the top catalyst and 690° F. for the bottom catalyst, and 6.3 MSCF/bbl H₂. Conversion below 650° F. was 58.2 weight %. The product was cut at about 300° F. and about 650° F. to give a diesel blend stock cut. Yields and diesel blend stock properties of this product are given in Table VI. As determined by ASTM D6468, the diesel blend stock was very stable. Aromatics in the diesel blend stock were 4.3 weight, olefins were 1.0 wt %, and oxygenates were 0.5 wt %. Paraffins are equal to the saturates (94.2%) because the GC-MS technique did not detect significant quantities of the sum of olefins and naphthenes. The Cetane Index was high (67.6) and the cloud point was −44° C. TABLE V FEEDSTOCK INSPECTIONS OF 700-1000° F. HYDROTREATED FT WAX Gravity, API 42.3 Sim. Dist., LV %, ° F. ST/5 691/804 10/30 824/884 50 919 70/90 940/974 95/EP  991/1031

TABLE VI HYDROCRACKING OF 700-1000° F. HYDROTREATED FT WAX OVER 3/1 NI—W—SIO₂—AL₂O₃/PT-SAPO-11 AT LHSV 1 hr⁻¹, 680° F./690° F., 300 PSIG, AND 6.3 MSCF/BBL Conversion <650° F., Weight % 58.2 Yield, Weight % C₁-C₂ 0 C₃-C₄ 4.78 C₅-180° F. 14.93 180-350° F. 15.53 350-650° F. 23.22 650° F.+ 41.92 C₅+ 95.7 350-650° F. Properties Weight % of Feed 31.1 Gravity, API 50.1 Viscosity, 40° C., cSt 2.027 Cloud Point, ° C. −44 Naphthenes + Olefins, Weight % (GC-MS) Not Detected SFC Analysis, Wt % Aromatics 4.3 Olefins 1.0 Oxygenates 0.5 Saturates 94.2 PNA Aromatics, weight % 0.5 % Reflectance, ASTM D6468 @150° C. 1.5 hr 99.2 3.0 hr 99.2 Cetane Index 67.6 Refractive Index @ 20° C. 1.4348 Density, g/ml @ 20° C. 0.7741 Molecular Wt. 196 Carbon types by ndM, weight % P/N/A 92.40/5.01/2.59 D2887 Dist., Wt %, ° F. ST/5 266/300 10/30 325/396 50 472 70/90 561/645 95/EP 667/698

The diesel blend stock of example 4 exhibited excellent stability as measured by ASTM D6468 at 150° C. for 180 minutes, as did the diesel blend stocks of examples 2 and 3, with results in excess of 99%. The polynuclear aromatic content of the diesel blend stock of example 4 was 0.5 wt %—less than 10 weight % of the total unsaturates (5.3%).

Example 5

A Low Temperature Fischer Tropsch wax and a Low Temperature Fischer Tropsch light condensate (Table VII) were processed over conventional hydroprocessing catalysts. 168 cm³/hr of the wax and a recycle liquid were hydrocracked over a 126 cm³ of sulfided NiW/acidic amorphous SiO₂-Al₂O₃ catalyst. 106 cm³//hr of the light condensate was mixed with the effluent from the hydrocracker and the mixture was hydrotreated over 68 cm³ of sulfided NiMo/non-acidic alumina catalyst. The effluent from the hydrotreater was distilled to obtain gas, naphtha, distillate fuel blend stocks, and the recycle liquid. Operating conditions, yields and product properties are summarized in Table VIII. The hydrocracking reactor is referred to as Rx 1 and the hydrotreating reactor as Rx 2. TABLE VII Properties of Fischer Tropsch Feeds Fischer Fisher Tropsch Fischer Tropsch Tropsch Wax Light Condensate Wax Feed for Example 5 5 6 Gravity, °API 40.3 53.5 Nitrogen, ppm 1.27 2.36 <0.25 D2887 Dist., Wt %, ° F. ST/5 451/573  91/206 441/545 10/30 623/725 253/345 587/694 50 790 434 786 70/90 870/973 519/625  880/1009 95/EP 1010/1068 651/702 1065/1161

Example 6

228 cm³/hr of a hydrotreated Low Temperature Fischer Tropsch wax (Table VII) was processed over 300 cm³ sulfided NiW/acidic amorphous SiO2-AlO3 catalyst in reactor 1 (Rx 1) and the combined effluent was processed over cm³ of a Pt/SAPO-11 catalyst in reactor 2 (Rx 2). The Pt/SAPO-11 was bound with 15 weight % alumina. The Pt/SAPO-11 catalyst is a complete hydroisomerization dewaxing catalyst. The product was distilled to obtain a diesel blend stock fraction. Yields, operating conditions and product properties are shown in Table VIII. TABLE VIII Properties of Diesel and Jet Blend Stocks at Different Pressures Example 15 15 15 15 16 Pressure, psig 1002 1002 297 297 298 Temperatures Rx 1/Rx 2, ° F. 675/600 675/600 666/600 666/600 600/685 LHSV Rx 1/Rx 2 1.3/4.0 1.3/4.0 1.3/4.0 1.3/4.0 0.76/2.3  Recycle gas rate, SCFB 5159 5159 4669 4669 4946 Per pass conv, LV % 77.8 77.8 77.9 77.9 — Recycle Cut Point, ° F. 702 702 696 696 — H2 Consumption, SCFB 461 461 429 429 260 Product Boiling Range, ° F. 300-700 300-555 300-700 300-555 300-700 API Gravity 55.7 55.2 49.5 Vis@40 C., cSt 2.055 2.237 2.817 Cloud, ° C. 2 2 −37 N, ppm <0.1 <0.1 <0.1 0.1 <0.1 S, ppm <1 <1 <1 <1 <1 SFC Analysis Aromatics, mass % 1.2 0.9 1.4 Olefins, mass % 5.4 4.8 2.6 Oxygenates, mass % 0.2 <0.1 <0.1 Saturates, mass % 93.2 94.3 96.0 JFTOT Test @ 260° C. passed passed JFTOT Test @ 300° C. passed passed D6468 Stability @ 150° C.  90 minutes 99.3, 99.6 99.8, 99.7 180 minutes 99.8, 99.8 99.8, 99.8 Peroxide Formation, ppm Week 0 1 1 <1 <1 Week 1 <1 1 <1 <1 Week 2 <1 1 <1 <1 Week 3 <1 1 <1 <1 Week 4 1 1.3 1 <1 Acid Number, mg KOH/g 0.06 <0.05 <0.05 D2887 Distillation by wt %, ° F. 0.5/5 287/304 285/302 293/309 289/305 285/326 10/20 343/384 310/346 344/385 321/348 357/422 30/40 420/455 382/392 421/465 385/397 476/525 50 489 422 516 424 562 60/70 523/565 451/463 548/586 455/467 593/623 80/90 603/652 491/521 625/663 492/522 652/679 95/99.5 689/761 525/547 685/729 525/548 692/708

Products made at 300 psig contained greater than 2 wt % unsaturates, more than 1 wt % olefins, had less than 1 ppm sulfur yet had excellent stabilities in diesel and jet fuel tests, and excellent resistance to peroxide formation. All products had greater than 90% paraffins. Products from the Pt/SAPO-11 catalyst showed a lower level of olefins presumably due to the presence of the more active Pt component.

The following are a series of Comparative Examples illustrating that untreated Fischer Tropsch products are unstable with respect to peroxide formation and that the conventional hydroprocessing operation yields a product with an extremely low amount of unsaturates which is stable with respect to peroxide formation.

Comparative Example 7

Preparation of a Fully Hydrogenated Diesel Blend Stock

A highly paraffinic diesel blend stock was prepared from three individual Low Temperature Fischer-Tropsch feedstocks. TABLE IX Properties of Fischer-Tropsch Feedstocks Property Feedstock 1 Feedstock 2 Feedstock 3 Wt % in blend 27.8 23.1 49.1 Gravity, °API 56.8 44.9 40.0 Sulfur, ppm <1 <1 Oxygen, ppm by Neut. Act. 1.58 0.65 Chemical Types, Wt % by GC-MS Paraffins 38.4 62.6 85.3 Olefins 49.5 28.2 1.6 Alcohols 11.5 7.3 9.3 Other Species 0.5 3.9 3.8 Distillation by D-2887, ° F. by wt % 0.5/5  80/199  73/449 521/626 10/30 209/298 483/551 666/758 50 364 625 840 70/90 417/485 691/791  926/1039 95/99.5 518/709  872/1074 1095/1184

The blend stock was prepared continuously by feeding the different feedstocks down-flow to a hydroprocessing reactor. The reactor was filled with a catalyst containing alumina, silica, nickel, and tungsten. The catalyst was sulfided prior to use. The per-pass conversion was maintained at approximately 80% below the recycle cut point of 665-710° F. by adjusting the catalyst temperature.

The product from the hydroprocessing reactor after separation and recycling of unreacted hydrogen was continuously distilled to provide a gaseous by-product, a light naphtha, a diesel blend stock fraction, and an unconverted fraction. The unconverted fraction was recycled to the hydroprocessing reactor. The temperatures of the distillation column were adjusted to maintain the flash and cloud points at their target values of 58° C. and −18° C., respectively.

The feedstocks were blended from several hours of consistent operation at 1.4 LHSV to provide the representative Product A in the Table X. TABLE X Properties of Blended Distillate Fuel Blend Stock Product Sample ID A B Gravity, °API 52.7 52.5 Nitrogen, ppm 0.24 0.25 Sulfur, ppm <1 0.61 Water, ppm by Karl Fisher, ppm 21.5 Pour Point, ° C. −23 −23 Cloud Point, ° C. −18 −18 Flash Point, ° C. 58 59 Autoignition Temperature, ° F. 475 410 Viscosity at 25° C., cSt 2.564 2.304 Viscosity at 40° C., cSt 1.981 1.784 Cetane Number 74 72.3 Aromatics by Supercritical Fluid <1 0.9 Chromatography, wt % Neutralization No. 0 Ash Oxide, Wt % <0.001 Ramsbottom Carbon Residue, wt % 0.02 Cu Strip Corrosion 1A Color, ASTM D1500 0 0.2 GC-MS Analysis Paraffins, Wt % 100 81.64 Paraffin i/n ratio 2.1 1.02 Oxygen as oxygenates, ppm <6 1226 Olefins, Wt % 0 17.52 Average Carbon Number 14.4 13.20 Distillation by D-2887 by Wt %, ° F. and D-86 by Vol %, ° F. D-2887 D-86 D-2887 D-86 0.5/5   255/300 329/356 256/298 334/360 10/20 326/368 366/393 329/367 366/—  30/40 406/449 419/449 400/429 413/—  50 487 480 463 466 60/70 523/562 510/539 500/537  —/519 80/90 600/637 567/597 574/605  —/572   95/99.5 659/705 615/630 626/663 587/604

Oxygen can be present in the sample in the form of organic oxygenates, measured by gas chromatography-mass spectrometry (GC-MS), dissolved or suspended water, measured by Karl Fischer, or dissolved O₂ from the air.

The oxygenate content was determined by GC-MS. Oxygenates in the sample were treated with tetraethoxysilane (TEOS) to increase the sensitivity of the technique. Oxygenates could not be detected Sample A. The limit of detection of the technique was determined to be 6.5 ppm per oxygenate. With the molecular weight range of diesel fuel this is equivalent to 0.6 ppm oxygen as oxygenates. Given that there are roughly 10 oxygenate compounds in a typical sample just below this limit of detection, the maximum amount of oxygen as oxygenates in the sample is 6 ppm (0.0006 weight %).

Using data from O₂ solubility in pure compounds it has been estimated that the solubility of O₂ from air in Product A is approximately 92 ppm (0.0092 weight %). There are no readily available methods for measuring dissolved O₂. The GC-MS analyses are shown in Table XI. TABLE XI GC-MS analysis of distillate fuel blend stock N-alkane Branched Formula area % alkane area % Total alkanes i/n by Carbon No. C₉H₂₀ 2.96 0.00 2.96 — C₁₀H₂₂ 3.59 4.24 7.83 1.18 C₁₁H₂₄ 3.80 4.65 8.45 1.22 C₁₂H₂₆ 3.65 4.77 8.42 1.31 C₁₃H₂₈ 3.41 5.34 8.75 1.57 C₁₄H₃₀ 3.00 5.34 8.34 1.78 C₁₅H₃₂ 2.61 5.56 8.17 2.13 C₁₆H₃₄ 2.33 8.65 10.98 3.71 C₁₇H₃₆ 1.99 5.74 7.72 2.89 C₁₈H₃₈ 1.51 6.11 7.62 4.04 C₁₉H₄₀ 1.60 5.98 7.58 3.73 C₂₀H₄₂ 1.18 5.35 6.53 4.52 C₂₁H₄₄ 0.58 3.82 4.41 6.54 C₂₂H₄₆ 0.22 2.00 2.23 8.94 Percent Paraffins 100.00 Percent Olefins 0.00 Average Carbon Number 15.12 Boiling point of Avg. Carbon No. ° F. 521 Total sample paraffin i/n ratio 2.08

As noted above, oxygenates were not detected in this Product A. Also, Product A contains less than 1 weight % aromatics. The lack of aromatics further increases the likelihood that Product A will rapidly oxidize.

Comparative Example 8

Preparation of an olefinic diesel fuel blend stock In this example, Feedstock 1 of the Fischer-Tropsch product in Table XI was by-passed around the hydroprocessing unit and fed directly to the distillation column. The same catalysts and conditions used in Example 7, including an LHSV of 1.4, were used, and the conditions of the distillation column were adjusted to maintain flash and cloud points in the product as used in Example 7. The yield of diesel fuel blend stock was less, near 73% due to requirement to reduce the end point of the diesel fuel blend stock to maintain cloud point.

Diesel fuel blend stock was blended from several hours of consistent operation to provide the representative Product B in the Table X. In contrast to the operation where all the Fischer-Tropsch streams were fed to the hydroprocessing unit, by-passing the light components resulted in lower yields of diesel fuel blend stock as a result of the lower diesel end point. The lower diesel end point was probably a result of the higher concentration of heavy n-paraffins in Product B. The GC-MS analysis of Product B are shown in Table XII. TABLE XII GC-MS analysis of Product B Carbon Paraffin No. 1-alkenes n-alkanes i-alkanes alcohols Sum i/n ratio C₆ 0.00 0.00 0.00 0.03 0.03 C₇ 0.00 0.00 0.00 0.21 0.21 C₈ 0.00 0.00 0.00 0.32 0.32 C₉ 2.49 2.49 2.13 0.21 7.32 0.86 C₁₀ 3.55 3.20 4.62 0.12 11.49 1.44 C₁₁ 3.91 3.91 4.97 0.03 12.82 1.27 C₁₂ 3.55 4.26 4.62 0.09 12.52 1.08 C₁₃ 2.35 4.36 4.69 0.00 11.39 1.08 C₁₄ 1.68 4.69 4.02 0.00 10.39 0.86 C₁₅ 0.00 4.36 6.03 0.00 10.39 1.38 C₁₆ 0.00 4.36 4.02 0.00 8.38 0.92 C₁₇ 0.00 4.36 3.35 0.00 7.71 0.77 C₁₈ 0.00 3.02 1.68 0.00 4.69 0.56 C₁₉ 0.00 1.34 1.01 0.00 2.35 0.75 Sums 17.52 40.32 41.14 1.02 100.00 Percent Paraffins 81.46 Percent Olefins 17.52 Average Carbon Number 13.20 Oxygen as oxygenates, ppm 1226 Total sample paraffin i/n ratio 1.02

These results also show that when a portion of the Fischer-Tropsch product by-passes the hydroprocessing reactor and is blended into the final blend stock, significant quantities of olefins are included in the blend stock product. The olefins in the blend stock product are in fact ten times greater than the alcohols. The olefins and oxygenates create potential stability problems.

Example 9

Stability Measurements

Product B was tested according to ASTM D6468 at 150° C. for 180 minutes and found to have a stability of 99.3%, which indicates that it is extremely stable towards deposit formation in this test.

The Products were then tested for peroxide formation under accelerated formation according to methods as described in U.S. Pat. Nos. 6,162,956 and 6,180,842. The Products were tested according to a standard procedure for measuring the buildup of peroxides. First, a 4 oz. sample was placed in a brown bottle and aerated for 3 minutes. An aliquot of the sample was then tested according to ASTM D3703 for peroxides. The peroxide content of the samples was measured by use of procedures following ASTM D3703 with exception that the Freon solvent was replaced by isooctane. The sample was then capped and placed into a 60° C. oven for 1 week. After this time the peroxide number was repeated, and the sample was returned to the oven. The procedure continued each week until 4 weeks have elapsed and the final peroxide number is obtained. Table XIII contains peroxide formation tendencies. TABLE XIII Peroxide Formation Tendencies A B Initial Peroxide No. 1.3 8.2 Peroxide No. after 1 weeks at 60° C. 1.0 35 Peroxide No. after 2 weeks at 60° C. 1.5 156 Peroxide No. after 3 weeks at 60° C. 1.88 204 Peroxide No. after 4 weeks at 60° C. <5 >5

An additional test of Product A was done at 70° C. The initial peroxide number and the peroxide number after 4 weeks are both less than 1 ppm. These results indicate that Product A has significantly better peroxide stability than Product B. These test results demonstrate the stability of fully hydrogenated low temperature Fischer Tropsch products and the very rapid peroxide forming tendencies of distillate fuel blend stocks containing unhydroprocessed Fischer Tropsch streams.

Example 10

Effect of Trace Olefins on Peroxide Stability

A further study was done to determine the effects of adding small amounts of olefinic condensate to the stable blend stock Product A of Table X. A 300-600° F. portion of the low temperature cold condensate, Feedstock 1 of Table IX, was obtained by distillation. The properties of the 300-600° F. portion of the cold condensate are as follows: TABLE XIV Properties of the 300-600° F. Portion of the Cold Condensate Property Value API Gravity 65.3 Nitrogen, ppm 0.79 Sulfur, ppm 2.29 Bromine No. 48.2 Simulated Distillation, D-2887 ° F. by Wt % 0.5/5% 296/302  10/30% 332/383 50% 393  70/90% 459/523  95/99.5% 551/654

A GC-MS analysis of the 300-600° F. portion of the cold condensate produced these results in weight %: TABLE XV GC-MS analysis of the 300-600° F. Portion of the Cold Condensate Carbon No. n-Alkenes Alkanes Alcohols Sum C₆ 0.00 0.00 0.00 0.00 C₇ 0.00 0.00 1.54 1.54 C₈ 0.00 0.00 0.32 0.32 C₉ 2.20 3.30 1.32 6.82 C₁₀ 12.37 5.35 1.03 18.75 C₁₁ 11.46 5.28 0.81 17.54 C₁₂ 10.37 5.94 0.54 16.85 C₁₃ 8.43 5.72 0.29 14.44 C₁₄ 5.85 4.69 0.19 10.74 C₁₅ 3.31 3.01 0.00 6.32 C₁₆ 1.60 1.76 0.00 3.36 C₁₇ 0.73 0.95 0.00 1.69 C₁₈ 0.34 0.55 0.00 0.89 C₁₉ 0.15 0.33 0.00 0.48 C₂₀ 0.06 0.21 0.00 0.26 Sums 56.87 37.10 6.03 100.00 Percent Paraffins 37.10 Oxygen as oxygenates, ppm 6769 Percent Olefins 56.87 Oxygen as primary C₁₂-C₂₄ 832 alcohols, ppm Average Carbon Number 12.03 Oxygen as primary C₇-C₁₂ 6398 alcohols, ppm Standard Deviation 2.10 Percent C₁₂-C₂₄ Material 55.02

Example 11

The 300-600° F. portion of the cold condensate was blended in varying amounts with the Stable Fuel Blend Stock A of Table X and the blends were evaluated for peroxide formation with the following results: TABLE XVI Peroxide Formation of Blends 1-5 Volume Peroxide Result vs Weeks of Sample Volume Cold Stable Fuel Olefins in the Storage at 60° C., ppm No. Condensate, ml ml Blend, Wt % 0 1 2 3 4 1 0 100 0 <1 <1 <1 <1 <1 2 0.2 99.8 0.1 <1 <1 <1 1.1 1.0 3 0.5 99.5 0.3 <1 <1 1.6 5.3 6.7 4 1 99.0 0.6 1.2 2.5 7.7 20.0 37.0 5 2 98.0 1.13 1.1 5.6 23.2 53.0 58.0

These results show that the blend stock prepared by hydrotreating the entire portion, without direct blending of cold condensate, is stable with respect to formation of peroxide. Blend stocks can tolerate only up to 0.2 weight % cold condensate (0.012 wt % oxygenates as alcohols determined by GC-MS and about 0.1 wt % olefins) and still be considered stable. Blend stocks with more than 0.012 wt % oxygenates or 0.1 wt % olefins did not exhibit satisfactory stability. As the oxygenate content was increased beyond 0.012 wt %, the peroxide stability of the blend stock rapidly declined.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof. 

1. A distillate fuel comprising a Fischer Tropsch distillate fuel blend stock, wherein the Fischer Tropsch distillate fuel blend stock comprises: a) unsaturates in an amount between 2 and 20 weight %; b) paraffins in an amount 80 weight % or greater; c) sulfur in an amount less than 1 ppm; d) a cetane index of greater than 60; and e) peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks.
 2. A distillate fuel according to claim 1, wherein the peroxide precursors are in an amount such that less than 4 ppm peroxides are formed after storage at 60° C. for four weeks.
 3. A distillate fuel according to claim 1, wherein the peroxide precursors are in an amount such that less than 1 ppm peroxides are formed after storage at 60° C. for four weeks.
 4. A distillate fuel according to claim 1, wherein approximately 100 weight % of the distillate fuel is a Fischer Tropsch distillate fuel blend stock.
 5. A distillate fuel according to claim 1, further comprising petroleum blend stock.
 6. A distillate fuel according to claim 5, wherein the fuel comprises 5 to 75 weight % Fischer Tropsch distillate fuel blend stock and 95 to 25 weight % petroleum blend stock.
 7. A distillate fuel according to claim 1, wherein the distillate fuel further comprises nitrogen in an amount less than 1 ppm.
 8. A distillate fuel according to claim 1, wherein the unsaturates are in an amount between 2 and 15 weight %.
 9. A distillate fuel according to claim 1, wherein the unsaturates are in an amount between 5 and 10 weight %.
 10. A distillate fuel according to claim 1, wherein the fuel conforms to at least one specification for either diesel fuel or jet fuel.
 11. A distillate fuel according to claim 10, wherein the distillate fuel conforms to at least one specification for a diesel fuel and has a cetane index greater than
 60. 12. A distillate fuel according to claim 11, wherein the distillate fuel has a cetane index greater than
 65. 13. A diesel fuel according to claim 11, wherein the fuel has a percent reflectance according to ASTM D6468 in excess of 65% when measured at 150° C. for 90 minutes.
 14. A diesel fuel according to claim 11, wherein the fuel has a percent reflectance according to ASTM D6468 in excess of 65% when measured at 150° C. for 180 minutes.
 15. A diesel fuel according to claim 11, wherein the fuel has a percent reflectance according to ASTM D6468 in excess of 99% when measured at 150° C. for 180 minutes.
 16. A distillate fuel according to claim 10, wherein the distillate fuel conforms to at least one specification for a jet fuel and has a passing rating in ASTM D3241 at 260° C. for 2.5 hours.
 17. A distillate fuel according to claim 16, wherein the fuel has a passing rating in ASTM D3241 at 270° C. for 2.5 hours.
 18. A distillate fuel according to claim 16, wherein the fuel has a passing rating in ASTM D3241 at 300° C. for 2.5 hours.
 19. A Fischer-Tropsch diesel fuel blend stock comprising: a) unsaturates in an amount between 2 and 10 weight % wherein the unsaturates comprise less than 10 weight % polynuclear aromatics; b) paraffins in an amount 90 weight % or greater; c) sulfur in an amount less than 1 ppm; and d) peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks, wherein the Fischer-Tropsch diesel fuel blend stock has a cetane index greater than 60, and a percent reflectance according to ASTM D6468 at 150° C. in excess of 99% when measured at 180 minutes.
 20. A blend stock according to claim 19, wherein the blend stock has a cetane index of greater than
 65. 21. A blend stock according to claim 19, wherein the blend stock has a cetane index of greater than
 70. 22. A Fischer-Tropsch jet fuel blend stock comprising: a) unsaturates in an amount between 2 and 10 weight % wherein the unsaturates comprise less than 10 weight % polynuclear aromatics; b) paraffins in an amount 90 weight % or greater; c) sulfur in an amount less than 1 ppm; and d) peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks, wherein the Fischer-Tropsch jet fuel blend stock has a smoke point of 30 mm or greater, and a passing rating in ASTM D3241 at 260° C. for 2.5 hours.
 23. A blend stock according to claim 22, wherein the fuel has a passing rating in ASTM D3241 at 270° C. for 2.5 hours.
 24. A blend stock according to claim 22, wherein the fuel has a passing rating in ASTM D3241 at 300° C. for 2.5 hours. 25-32. (canceled)
 33. A distillate fuel comprising a Fischer Tropsch distillate fuel blend stock, wherein the Fischer Tropsch distillate fuel blend stock is made by a process comprising: a) converting syngas to a Fischer Tropsch derived feedstock by a Fischer Tropsch process; b) hydroprocessing the Fischer-Tropsch derived feedstock at a temperature of 525-775° F., a pressure of less than 1000 psig, and a liquid hourly space velocity of greater than 0.25 hr⁻¹; and c) recovering a Fischer Tropsch distillate fuel blend stock, wherein the Fischer Tropsch distillate fuel blend stock comprises between 2 and 20 weight % unsaturates, less than 1 ppm sulfur, and peroxide precursors in an amount such that less than 5 ppm peroxides are formed after storage at 60° C. for four weeks.
 34. A distillate fuel according to claim 33, wherein the fuel conforms to at least one specification for either diesel fuel or jet fuel.
 35. A distillate fuel according to claim 34, wherein the fuel conforms to at least one specification for a diesel fuel and has a cetane index greater than
 60. 36. A distillate fuel according to claim 34, wherein the fuel conforms to at least one specification for a diesel fuel and has a cetane index greater than
 65. 37. A distillate fuel according to claim 34, wherein the fuel conforms to at least one specification for a jet fuel and has a passing rating in ASTM D3241 at 260° C. for 2.5 hours. 38-41. (canceled) 